Why do plants need a transport system?
- To provide a way of carrying water and ions from the root to the leaves.
- To provide a way of carrying sugars and amino acids from the leaves to other parts of the plant.
- If necessary, from storage organs to the leaves when it is not photosynthesizing.
What tissues of the plant make the transport system?
The two vascular tissues of flowering plants involved in transporting materials around the plant are:-
1. Xylem – transports water and dissolved minerals from the roots up the stem and leaves. (Also xylem gives support to the plant).
2. Phloem – transport food (mainly sugar and amino acids)
Vascular tissues extend uninterrupted from the roots to the stem and leaves in a typical dicotyledonous plant are shown below
Transpiration is the evaporation of water at the surfaces of the mesophyll cells followed by the loss of water vapour from the leaves through the stomata.
Position of xylem and phloem in the vascular bundle of a dicot root, stem and leaves in a typical dicotyledonous plant are shown below.
Structure of xylem
Characteristics of xylem vessel include:-
a. They are long and narrow cells joined end to end
b. They are hollow and made up of dead cells, therefore no protoplasm.
c. The cells do not have cross walls and continuous tubes from roots to leaves
d. The cellulose cell wall is strengthened by lignin.
e. Xylem vessel cannot become turgid as they do not have cell membrane and cytoplasm.
Lignin is a tough, complex organic compound that gives a plant its woody characteristic.
Lignin can deposit in many forms:-
TRANSPORT OF WATER AND MINERAL ION IN PLANTS (ASCENT OF SAP)
The absorption of water by root hair cells.
The root hair cell is an out growth of root epidermis. It increases the surface area for the absorption of water and mineral ions. In between the soil particle there is a thin film of water. This water has more water potential than the cytoplasm of the root hair cell. Cell membrane is partially permeable. There fore water is moved into the cytoplasm by osmosis
Water enters to the root hair cells by osmosis and diffuses through the root cortex cells until it reaches the xylem vessel which is located at the centre of the root. As the result of entry of more water in to the root cells, it creates a pressure in the cells. This pressure forces water into the xylem. This pressure is known as the root pressure
Movement of minerals in the root
Mineral salts are dissolved in soil water and exist in the form of ions. They are often present in low concentration. This indicates that active transport occurs in the uptake of ions. Special carrier molecules in the cell membrane carry the ions into the cell against the concentration gradient. Energy use during this transport is supplied by the mitochondria in the root.
Three forces that cause water to rise up a plant
1. Transpiration pull
Due to transpiration the water pressure at the top of the xylem tube is very low whereas at the bottom is very high. Therefore water is lifted up by a lifting force. This is called transpiration pull. As a result a continuous stream of water passes through the narrow capillary tube. This is called Transpiration stream. In addition to this capillarity, root pressure, cohesive (attraction between two water molecules) and adhesive forces (the tendency for water molecules to stick to the surface of xylem vessel) also help the water to lift up through the narrow capillary tube
2. Root pressure
Pressure exerted in the xylem vessel due to the inward movement of water in the root system which result in entry of water to the xylem vessel and move upward. Root pressure can be observed when water continues to exude from a freshly decapitated stump of a plant as seen here:-
Capillarity is the capacity of the water to move through a capillary tube against the force of gravity. Xylem vessels have a microscopic diameter which can move water 20 cm or more upward. This force can be demonstrated by dipping a very narrow tube into water. Due to adhesion force, the water molecules stick to the surface of the xylem and are pulled along the tube.
Definition: – the evaporation of water at the surfaces of the mesophyll cells followed by the loss of water vapour from the leaves through the stomata.
How transpiration occurs
The outer surface of the mesophyll cells are lined by a thin film of water. This water is necessary for the exchange of gases. This water is evaporated into the airspaces. To replace this water, more and more water move out of the mesophyll cells. As a result the water potential of the mesophyll cells decreases. The water potential of the water in xylem vessel is more than that of the cytoplasm. Therefore water from the leaf xylem move into the mesophyll cells by osmosis.
When the humidity inside the airspace becomes more than outside, the water vapour diffuses out through the open stomata. This is called transpiration
If the transpiration rate is higher than the rate of water uptake from the soil, the cells lose their turgidity and the plant wilts. Transpiration occurs whenever the stomata are open for the intake of carbon dioxide.
Opening and closure of stomata
Various theories have been proposed as to how stomata open and close. Stomata open in the presence of light.
- When light is present, the guard cells photosynthesis and glucose concentration increases, also potassium ions move into the guard cell and lowers the water potential inside the cells.
- Due to this, osmosis occurs and water moves into the guard cell. The cells become turgid and expand.
- Since the dorsal wall (outer wall) of the guard cells are thinner than the ventral side (inner wall), the outer wall expands more so the cells curve up opening the stomata wider.
In the darkness glucose and potassium ions move out of the guard cells lowering the water potential of the surrounding cells and water leaves the guard cells by osmosis. The guard cells become less turgid and change shape, closing the stomata.
Conditions that affect the rate of transpiration
As the temperature increases, more water evaporates into the air space inside the leaf; the molecules get more energy and move faster, this increases the humidity or water potential inside the leaf. Also increasing temperature lowers the relative humidity outside the leaf. Both these events results a steeper water potential gradient. The steeper the gradient, the faster the rate of diffusion.
Humidity (amount of water molecules in the air)
As humidity outside the leaf increases, the rate of transpiration decrease. When humidity increases, more water vapour is present outside the leaf than the air space inside the leaf. This reduces the steepness of the water potential gradient between inside and outside the leaf. So the rate of transpiration decreases.
Moving air (wind)
In still air, a highly saturated air builds up around the leaf reducing the steepness of the diffusion gradient between the atmosphere inside the leaf and outside. Any air movement will tend to sweep away this layer. Thus windy condition results in increased transpiration rate.
Light affects transpiration because stomata usually opens in the light and closes in darkness. As the light intensity increases the degree of opening of stomata also increases, hence transpiration also increase. (Light does not increase transpiration rate directly, it increases the degree of stomata opening).
Some internal factors
- Cuticle – the cuticle contain a fatty substance called cutin, which is water proof. This reduces evaporation of water from the leaves.
- Stomata – the greater the number of stomata per unit area the greater the rate of stomatal transpiration
Advantages of transpiration
1. It maintains a constant supply of ions to the leaves.
2. It brings water to the mesophyll cells for photosynthesis.
3. Supply water to all the cells for metabolic and processes and for turgidity
4. Evaporation of water helps to cool the leaves
Disadvantages of transpiration
1. Excessive transpiration leads to the wilting of the plants.
2. Large amount of potable water is absorbed from the soil and evaporated into the atmosphere. This reduces the stock of potable water in the soil.
Transpiration is sometimes referred as the ‘necessary evil’ because of its good and bad effects on plants.
Some adaptations plants have in order to minimize transpiration.
- Curled up leaves, so it creates a humid environment around the leaf, hence less transpiration occurs.
- Presence of hair like structures in the leaves, to trap escaping water molecules, to keep a humid environment around the leaf.
- Sunken stomata
- Having stomata closed during the day time
- Multi layered epidermis.
Measuring the rate of water uptake in plants
Transpiration increase when a large surface area of the leaf is exposed to air. The higher the rate of transpiration, the faster the rate of water uptake. A plant with more number of leaves will have higher rate of transpiration and faster uptake of water than a plant with less number of leaves.
Potometer is the instrument used to measure the rate of water uptake in a plant. It can come in many shapes. Basically it is an apparatus that is used to measure the rate of water uptake in a leafy shoot. When a potometer is used to calculate the rate of transpiration, it is assumed that all the water taken up is lost by transpiration.
The weighing method.
The volume of water intake at a given time in the following apparatus can be calculated by weighing the apparatus at the beginning and after the experiment. The difference shows the amount of water taken in by the plant.
Translocation of food
Transport of chemicals around the plant is translocation; usually it is transport of sugar and amino acids from the site of production to other areas of the plant.
Structure of phloem
Phloem consists of sieve tubes and companion cells. Sieve tubes are plant cells without a nucleus but companion cells have nucleus.
Some of the characteristics of these conducting cells are
1. They have cytoplasm but no nucleus
2. The cells are joined end to end and their walls are perforated known as the sieve plates.
3. Strands of cytoplasm extend through the pores of sieve plates into the next cell.
How food is transported down the phloem
From the leaves to other parts of the plant food is transported by bulk flow down a pressure gradient. Sugar and other products of photosynthesis are taken into the phloem tissue by active transport and water moves into the phloem from xylem by osmosis, due to this, pressure increases inside the phloem cells and, it pushes the flow down, hence the bulk flow.
The Girdling experiment
This experiment proves the phloem tissue is found closer to the surface of the stem and also sugar is transported through phloem.
If a ring of bark from the stem is removed, the phloem cells are also removed. The sugar transported in the phloem then gets accumulated and the area above the cut swells.
Experiment using radioactive carbon dioxide
A plant is supplied with carbon dioxide in which carbon is radioactive. Keep the plant in sunlight for minimum 6 hours. Plant uses this carbon dioxide and produce carbohydrate in which the carbon atom is radioactive. Take a cross section of the stem and placed on a photographic plate in a dark room. Black patches can be seen on the photographic plate where phloem was in contact with the plate.